(1) Field of the Invention
The present invention relates to a developing device, an imaging unit and an image forming apparatus for developing a latent image on an image carrier with use of developer.
(2) Related Art
A developing device including a housing, a developing roller as a developer carrier, and so on is known as a developing device in an image forming apparatus such as a copying machine. The housing contains therein powdery developer including particles with magnetic properties, and has an opening at a side facing a photosensitive drum as an image carrier. The developing roller is provided at the opening of the housing, and rotates to convey the developer while holding the developer on an outer circumferential surface thereof.
One problem of such a developing device using the powdery developer is that there are toner particles moving away from the developing roller without being provided for development and floating around the developing roller, although the amount thereof is small. Once the floating toner is carried by air currents in the device and leaks from axial ends of the developing roller to the outside of the housing, the leaking toner is likely to make an interior of the apparatus dirty. The developer can also leak from the axial ends of the developing roller to the outside of the housing.
In order to address the above-mentioned problem, there has been used a structure in which a magnetic member elongated along a rotational direction of the developing roller is provided so as to surround an outer circumferential surface of each axial end portion of the developing roller with a predetermined gap therebetween. With this structure, a magnetic blush of the developer is generated between the developing roller and the magnetic member. As the generated magnetic blush serves as a barrier, the floating toner or the developer is prevented from leaking to the outside.
In the art of an image forming apparatus, there is a strong demand for miniaturization of an image forming apparatus. This includes reduction of a length of a developing roller in a developing device in an axial direction.
A developing width of the developing roller (a length of an effective developing region in the axial direction of the developing roller) is determined in advance based on a maximum paper size and so on, and thus it is impossible to reduce the developing width. Therefore, it is required to reduce a length of a region other than the effective developing region in the axial direction of the developing roller.
For example, the developing roller is made longer than the developing width to provide, at each end portion thereof, the magnetic member for generating a magnetic blush between each axial end portion of the developing roller and the magnetic member. By reducing the lengthened part of the developing roller, the length of the developing roller itself can be reduced.
However, the reduction of the lengthened part of the developing roller means reduction of the width of the magnetic member in the axial direction of the developing roller. If the width of the magnetic member is reduced, a width of a region where the magnetic blush is generated throughout the magnetic member is also reduced. With this structure, leaking of the floating toner might not be prevented.